This invention relates generally to rotary dryers, and more particularly, to an improved means and method for controlling the transport rate of the bed in such dryers.
Rotary dryers are widely used for thermally processing and drying coarse- and fine-grained solids. Rotary dryers generally include a hollow cylindrical shell with open ends which is mounted for rotation about its axis. The axis may be slightly inclined from the horizontal plane. The interior surface of the shell may be provided with flights or lifters for elevating and showering the solids as the dryer rotates. The showering affords intimate contact of the solids with the gases which are channelled through the cylindrical shell.
Coolers are similar to rotary dryers, but opposite in function. The dryers are supplied with heated air to volatize moisture or solvent from the solids in the dryer bed, whereas coolers are supplied with unheated or cooled air to extract excess heat from the hot solids. In other respects, the relationships governing the heat transfer and bed transport rate through the shell are substantially similar.
Air flow through rotary dryers or coolers may be co-current or counter-current in relation to the direction of solids transport. Mathematical relationships which govern bed transport and heat transfer in rotary dryers have been published in the technical literature. For example, see the article appearing in Chemical Engineering Progress, September 1954, pp. 467 - 475, by W. C. Saeman and T. R. Mitchell, and also an article appearing in Chemical Engineering Progress, June, 1962, pp. 49 - 56, by W. C. Saeman.
According to the relationship proposed in the paper by Saeman and Mitchell, the rate of bed transport in a rotary cylinder equipped with flights and operating with counter-current air flow may be expressed as follows: EQU V = CDR(S - mA)
wherein V equals the horizontal transport velocity in feet per minute, C equals a constant with an assigned value in the range of 2 to 3 (usually 2.6), D equals diameter of the cylinder in feet, R equals rate of rotation in revolutions per minute, S equals slope of the axis in feet per foot of axial distance, m equals a coefficient to relate air velocity effects to equivalent slope effect, and A equals air velocity in units compatible with the coefficient m.
If air velocity is expressed as feet per second, the value of the coefficient m is in the range of 0.003 to 0.010. In cases where the cylinder is operated with co-current air flow, the factor (S - mA) is generally replaced with (mA + S), although in some instances (mA - S) may also apply.
In typical cases, the air flows through dryers and coolers in the range of 3 to 10 feet per second. The value of the term mA with m equal to 0.005 would thereupon be in the range from 0.015 to 0.05. Thus, assuming the case of a cylinder 10 feet in diameter and rotating at 10 rpm, even with zero slope (S = 0) the rate of bed transport by co-current air transport alone lies in the range of 3.75 to 12.50 feet per minute when the value of the coefficient C is 2.5. A dryer 40 feet long would then be transversed in about 3 to about 10 minutes.
The required retention time in dryers varies with different materials and typically ranges from 10 minutes to over 100 minutes. It is thus apparent that where the desired retention time is greater than that allowed by the bed transport rate, some means of retarding bed transport must be applied to diminish the relatively rapid air transport rate.
One common practice used to retard the bed transport rate is to operate the dryer with an axial slope in such a direction that it opposes the air transport effect so that the net bed transport rate is the difference between the two. Thus, if a negative slope of 0.01 is combined with the mA value as indicated above, the values of the combined factors (mA - S) for the conditions specified above becomes 0.005 and 0.040, thereby reducing the bed transport rates from 3.75 and 12.50 feet per minute to 1.25 and 10 feet per minute at co-current air velocities of 3 and 10 feet per minute respectively. At an air velocity of 3 feet per second, bed retention time is thereby extended from 10 minutes to over 30 minutes, while bed retention is extended only slightly from 3 to 4 minutes at the higher air velocity. It is therefore apparent that this method of bed transport retardation is effective only if the slope of the cylinder is readjusted in conformity with the term (mA - S) or (S - mA) for any particular value of air rate, or vice versa, as the bed transport rate is proportional to the difference between the two terms. Because of this relation, variations in air rate are inflexibly tied to variations in the axial slope. However, because of the massiveness of industrial rotary dryers, the slope cannot be conveniently altered, hence the air rate must be held within relatively narrow limits of variation, which is difficult in many instances.
Another way to reduce the bed transport rate would be to reduce the speed of rotation of the drum. However, in the publication by Saeman and Mitchell mentioned above, it is demonstrated that the heat transfer rate between air and solids is proportional to the rate of rotation of the cylinder. Thus, reducing the speed of the rotation of the drum to reduce bed transport rate will penalize the rate of heat transfer in the drum. This would necessitate the use of larger, low-speed dryers to yield the same heat transfer capacity achieved in smaller, high-speed dryers.
Aside from variations in cylinder slope and rotational speed to regulate the rate of bed transport in rotary cylinders, it has also been customary to install dam rings in suitable positions in the interior of the cylinder to force the accumulation of the bed behind the dam ring until the bed level reaches the overflow point of the ring. This again leads to a relatively inflexible situation in that the volume of the retained bed behind the dam ring remains fixed by the geometry of the dam rings regardless of drum speed, air flow rate, slope, or dryer feed rate. There will be longer retention times at low feed rates and shorter retention times at high feed rates. Such as relationship could be detrimental to the drying of thermally sensitive material for which the retention time under all circumstances of operation should be held at a minimum. Excessive retention time is also wasteful on the cylinder driving power, since unnecessary bed retention adds an additional load to the cylinder drive.
A further disadvantage of the dam ring method of bed rate control is the fact that bed depth is not constant, but diminishes progressively in a direction upstream from the dam ring. The diminishing bed depth results in a decrease of the heat transfer coefficient between the gases in the dryer and the solids.
In view of the above, it is an object of the present invention to provide an improved rotary dryer.
A specific object of the present invention is the provision of a rotary dryer wherein the bed transport rate is relatively insensitive to changes in air rate, rotational speed, and slope.
Another object of the present invention is the provision of a rotary dryer wherein the bed transport rate may be readily changed irrespective of slope, cylinder speed, or air rate.
A further object of the present invention is the provision of a rotary dryer wherein the distribution of the bed upstream from the dam ring is equalized to effect an equilizatin of the heat transfer coefficient.
A still further object of the present invention is the provision of an improved combined rotary dryer-cooler which utilizes co-current drying gas and counter-current cooling gas.
According to the present invention, there is provided a rotary dryer comprising a cylindrical shell, the interior of which is provided with at least one set of circumferentially spaced flights and a dam ring. The dam ring is spaced downstream of the flights to provide a flight-free area. Means are provided capable of transporting material from the flight-free area over the dam ring to the downstream side thereof and which is adjustable to vary the amount of material retained upstream from the dam ring.
According to a further form of the invention, means may also be provided to convey material from the flight-free zone to a point adjacent the upstream end of the flights.
In addition, the cylindrical shell may be provided with a cooling section comprising a plurality of flights. Means for supplying co-current drying gas is provided at the feed end of the dryer and means for supplying cooling gas is provided adjacent the discharge end.